The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 33

separate chapter) but the audio data rate is always constant and a multiple
of 8 kbit/s. The compression itself will be dealt with in a separate chapter.
The compressed video and audio signals in MPEG are called “elementary
streams”, ES in brief. There are thus video streams, audio streams and,
quite generally, data streams, the latter containing any type of compressed
or uncompressed data. Immediately after having been compressed (i.e. en-
coded), all the elementary streams are divided into variable-length packets,
both in MPEG-1 and in MPEG-2 (Fig. 3.3.).

var. length
up to 64 kbyte
at MPEG1

Video PES

PES packet

PES header

Audio PES

Data PES

Fig. 3.3. MPEG Elementary Streams

Since it is possible to have sometimes more and sometimes less com-
pression depending on the instantaneous video and audio content, variable-
length containers are needed in the data signal. These containers carry one
or more compressed frames in the case of the video signal and one or more
compressed audio signal segments in the case of the audio signal. These
elementary streams (Fig. 3.3.) thus divided into packets are called “pack-
etized elementary streams”, or simply PES for short. Each PES packet
usually has a size of up to 64 kbytes. It consists of a relatively short header
and of a payload. The header contains inter alia a 16-bit-long length indi-
cator for the maximum packet length of 64 kbytes. The payload part con-
tains either the compressed video and audio streams or a pure data stream.
According to the MPEG Standard, however, the video packets can also be
longer than 64 kbytes in some cases. The length indicator is then set to